I. Field
The present disclosure relates generally to electronics circuits, and more specifically to output circuits for electronics devices.
II. Background
Output amplifiers are commonly used in various applications such as audio power amplifiers, telephone line drivers, etc. Output amplifiers may be categorized into different classes such as class A, class B, class AB and class D. Class A, B and AB amplifiers are linear amplifiers that operate in linear region. Class D amplifiers are switch mode amplifiers that operate in triode and cut-off regions. Consequently, class D amplifiers can typically achieve much higher power efficiency than linear amplifiers.
A class D amplifier is commonly used with a modulation technique such as pulse width modulation (PWM) or pulse density modulation (PDM). A PWM modulator may receive an analog input signal and a carrier signal and generate a PWM signal that controls the switching of the class D amplifier. The PWM signal has a variable duty cycle determined by the amplitude of the analog input signal and a fixed switching frequency determined by the frequency of the carrier signal. The carrier signal may have a sawtooth waveform or a triangular waveform. Any deviation from an ideal sawtooth or triangular waveform may cause distortion in an output signal from the class D amplifier.
A PDM modulator may receive an analog input signal and generate a PDM signal that controls the switching of the class D amplifier. The PDM signal has a duty cycle determined by the analog input signal and may have a variable switching frequency determined by circuit components within the PDM modulator. The values of the circuit components may vary widely due to integrated circuit (IC) process variations and other factors. Hence, the switching frequency of the PDM signal may also vary widely.